Improvements to the vector module: Del and Orienters

1. Introduces the Orienters API for good code structure
2. Del class now returns unevaluated expressions(vectorial or scalar) composed of Derivative instances. The user can call .doit() to evaluate them.
3. adds the following printing implementations for the vector module - pretty, latex
4. scalar\_map method now takes into account the relative location of one coordinate system wrt another.
5. adds the methods 'measure\_numbers()' to Vector
6. Adds helper methods for calculating curl, divergence, gradient. Also adds the scalar\_potential and scalar\_potential\_difference functions present in the physics.vector module
I wonder if we should even support this. We could just show an example of subclassing the vector printer or something.